plugin/health: close codeblock
plugin/health: close codeblock

Codeblock wasn't properly closed in the README.

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>

ii/coredns,coredns/coredns,miekg/coredns,jremond/coredns,jremond/coredns,yongtang/coredns,miekg/coredns,johnbelamaric/coredns,johnbelamaric/coredns,chrisohaver/coredns,ii/coredns